Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, as well as NOx emissions. In addition, fuel economy is improved, especially at higher levels of engine boost.
Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Additionally, internal combustion engines often include a turbocharger assembly. The turbocharger assembly uses the flow of exhaust gas to spin a turbine, which in turn drives a compressor that compresses the combustion air that is supplied to the intake manifold. When the exhaust gas from a predetermined number of the cylinders of the internal combustion engine is dedicated to the intake manifold for EGR purposes, thereby bypassing the turbocharger assembly, the flow rate of the exhaust gas available to the turbine of the turbocharger is reduced, which reduces the maximum power output of the internal combustion engine. In addition, the engine may suffer from turbo lag.
One example of a dedicated EGR cylinder system where this boost issue is addressed is shown by Hayman et al. in U.S. Pat. No. 8,539,768. Therein, the turbocharger assembly includes a bypass valve selectively coupling a dedicated EGR cylinder group to an exhaust turbine. During conditions when higher boost is required, the bypass valve may be opened so that exhaust gas from dedicated EGR cylinders can be used in combination with exhaust gas from remaining engine cylinders to spin the exhaust turbine. In comparison, during conditions when boost demand is lower, the bypass valve may be closed so that exhaust gas from the dedicated EGR cylinders is only used for EGR purposes and only the exhaust gas from the remaining engine cylinders is used to spin the exhaust turbine. In still other engine systems, turbo lag may be addressed through the use of air that is blown through one or more cylinders operating with valve overlap. By concurrently adjusting (e.g., enriching) the fueling of the cylinders operating with valve overlap, an amount and temperature of charge delivered to the turbine can be raised, thereby expediting turbine spin-up.
However, the inventors herein have identified potential issues with such approaches. As an example, in engine systems operating with a blow-through mode, the eventual flow of the blow-through air over the exhaust catalyst can lead to a drop in catalyst efficiency and exhaust emissions issues. As another example, in engine systems operating with a bypass valve selectively coupling the dedicated EGR cylinder group to the exhaust turbine, the use of EGR is limited to conditions when boost demand is low. In other words, high EGR availability and high boost availability may be mutually exclusive since the exhaust gas from the dedicated cylinder group can either be routed for EGR purposes or routed for turbine spin-up purposes. As such, EGR may be desired at higher engine boost levels to improve fuel economy and reduce NOx emissions.
In one example, the above issues may be at least party addressed by a method for an engine, comprising: operating a dedicated EGR cylinder group with rich cylinder combustion and more blow-through air than remaining engine cylinders, the dedicated EGR cylinder group recirculating exhaust gas to an engine intake via a first turbine, the first turbine distinct from a second turbine receiving exhaust gas from remaining engine cylinders. In this way, an exotherm can be generated at the turbine downstream of the dedicated EGR cylinder group, and boost development can be expedited.
As an example, exhaust from a dedicated EGR (DEGR) cylinder group of a multi-cylinder engine may be passed through a water gas shift (WGS) catalyst and then through a first, smaller exhaust turbine before the exhaust is recirculated to all engine cylinders. Exhaust from the remaining engine cylinders, in comparison, is passed through a second, larger turbine before being released through the tailpipe. Fueling of the DEGR cylinder group may be enriched during conditions when engine combustion stability is limited so that the WGS catalyst can create hydrogen-enriched exhaust for recirculation to the engine. The first turbine may drive a first, smaller compressor that is positioned upstream of a second, larger compressor, the second compressor driven by the second turbine.
In response to an operator pedal tip-in event, an amount of blow-through air delivered to the DEGR cylinder group may be selectively increased. Specifically, a variable cam timing device may be actuated, such as a faster actuating electric cam phaser, to adjust a timing of the intake and exhaust valves of only the DEGR cylinder group to a timing that provides increased positive intake to exhaust valve overlap (e.g., full valve overlap). At the same time, fueling of the DEGR cylinder group may be adjusted based on the blow-through air amount to provide an overall rich combustion. Alternatively, one or more post fuel injections may be added (e.g., in the exhaust stroke of the given combustion event). The degree of richness may be adjusted based on the boost demand relative to the boost pressure (or turbine speed) at the time of tip-in. In doing so, an exotherm is generated at the first turbine, expediting turbine spin-up. By spinning up the first turbine, boost provided by the first and second compressors may be increased, reducing turbo lag. The operation of the DEGR cylinder with increased blow-through air and increased richness of combustion may be continued until the first turbine is sufficiently spun up, or boost pressure is sufficiently high. Thereafter, the blow-though air amount may be reduced and DEGR cylinder group fueling may be adjusted based on engine operating conditions including EGR demand and combustion stability.
In this way, an engine configuration is provided where blow-through air delivery to a dedicated EGR cylinder group can be selectively and transiently increased. By concurrently using rich fuel injection, an exotherm can be generated at a turbine downstream of the DEGR cylinder group, expediting boost pressure development. Since the exhaust from the DEGR cylinder group is delivered to the engine intake, and not the engine exhaust, the blow-through air does not degrade the exhaust catalyst efficiency, thereby averting emissions issues otherwise associated with the use of blow-through air. By using the exhaust energy from a dedicated EGR cylinder group to run a dedicated turbine, exhaust energy from the dedicated cylinder group can be efficiently captured for creating boost, even at lower engine speeds. In addition, a smaller turbine can be used to reduce turbo lag. The use of a smaller turbine wheel results in less inertia, thereby allowing the maximum boost output to be achieved faster.
Furthermore, the reduction in exhaust temperature across the turbocharger results in lower temperature EGR being recirculated, which is advantageous for slowing combustion and controlling knock on cylinders ingesting the EGR. By using rich exhaust from a dedicated EGR cylinder to spin up a smaller turbine while stoichiometric exhaust from remaining engine cylinders to spin up a larger turbine, turbo lag can be reduced during a tip-in. By recirculating the rich exhaust from the dedicated EGR cylinder to remaining engine cylinders after the exhaust flows past the first turbine, EGR can be provided to the engine even during higher boost levels without degrading combustion stability. Overall, turbo lag can be rapidly reduced without affecting engine exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.